Fix spelling error

Missing 'n'.
Specify that Bootstrap is a front-end framework

Taken from the Bootstrap homepage: "Sleek, intuitive, and powerful front-end framework for faster and easier web development."

Nitpick. There's no such thing as a "UX framework", and Bootstrap also provides more than just UI widgets, so I figured it would probably be best to just use the same description Twitter chose.
Optimised images

Originally from https://github.com/meteor/meteor/pull/569

Reworked to:
- apply to soma.png, not soma.jpeg (file was previously misnamed).
- don't touch bootstrap's glyph image. It would cause conflicts later, and is not neighborly.
Add underscore as dependency for Spark

Fixes #587
Added Meteor._debug function to dependency list
Merge branch 'pr-588' into devel